The present invention relates to screening devices for separating material using vibratory motion to enhance separation. Specifically, the present invention relates to screening devices of light construction using vibratory motion to enhance separation of materials.
Screening devices are used in a number of industries to separate a variety of products based on size. Separation, sifting, and the like through screens have long been accomplished with the assistance of vibratory motion.
A variety of vibratory motions have been employed in screening devices. High-frequency vibration is used in some devices, wherein the screen is vibrated on the order of 10,000 Hz and responds through rapid particle separation. Typically, these devices also utilize low-frequency vibration to assist in transporting and dispersing material across the screen; usually, larger material that travels across the screen unfiltered is collected. Another form of vibratory motion used is low-frequency vibration, typically produced through counter-rotation of eccentric weights. Finally, some devices employ a method wherein the screen is vibrated at a frequency at or near one of its resonant frequencies, and is tuned closer and closer to that resonant frequency. This method advertises a low input power requirement because of the efficient nature of the resonant vibration, but requires massive components and foundation for resonant damage resistance. Similarly, the above-discussed low and high frequency vibration devices are constructed of heavy components and foundations since they cannot guarantee that their operating frequency range will not coincide with any of the device""s resonant frequency modes. These heavy construction setups require more power to impart a selected vibration regime than would a lighter construction.
Vibrating screen machines for use in the mineral processing industries are commonly used to separate minerals such as coal or ores by size, usually after crushing. The apparatus generally comprises RHS or boxed I-beam cross members spacing apart a pair of side walls. The cross members support a screen panel assembly of spaced apart screen support members for supporting the ends of modular screen inserts and intermediate stringer members mounting the screen support members to the cross members. The side walls are further interconnected by an upper box-section cross beam which serves as a mount for exciter units. The upper end of the assembly includes a feed box that doubles as a further cross member. The screen panel may be flat or may be curved to form a so-called banana screen.
The apparatus is engineered massively to resist the damage occasioned by the vibrating action of the exciter causing resonance with at least one of the multiple modes of vibration of the apparatus in use. The apparatus is generally engineered such that the side walls and RHS or boxed I-beam cross members in assembly are of massive construction to stiffen out all reasonable modes of destructive vibration. Since the side walls are of plate construction they are reinforced to close out destructive vibration, and the upper box-section cross beam and feed box are also heavily engineered to rigidly restrain the side walls one relative to the other to stiffen the apparatus overall. In essence, the philosophy is to pursue robustness at the expense of weight.
Heavy construction carries some inherent disadvantages that have hitherto been accepted. The first is that the mass of the machine affects the construction and shipping costs. The number of bolt fixings necessary to assemble the massive structure adds to construction costs also. The complexities of screen mounting occasioned by the heavy cross members under the screen panel requiring stringer bars and screen mounting bars is also a problem that has hitherto been accepted as a cost of imparting the requisite robustness to the structure.
The subject invention provides new lightweight vibratory screening devices for use in separating items of different sizes.
In a preferred embodiment, the apparatus has a pair of opposed side walls, exciter supports on an upper portion of each of the side walls, an exciter assembly mounted on the exciter supports, a torsion member secured between the side walls, screen panel support members disposed between the side walls, and screen panels adapted to attach to screen panel support members. In a specific embodiment, the side walls"" profile and stiffness, in addition to the torsion member dimensions are selected and adjusted such that the closest resonant modes of the side walls below and above the device operating frequency are separated by at least about 4 Hz.
Specifically exemplified herein are embodiments when the device is designed and adjusted such that the first fundamental frequency mode greater than the frequency of operation is at least about 2 Hz greater than the frequency of operation and the first fundamental frequency below the frequency of operation is at least about 2 Hz lower than said frequency of operation.
The side walls profile may be selected and adjusted, for example, by stiffeners positioned along the upper edge, lower edge, and middle section of side walls. The torsion member can be selected and adjusted, for example, by varying the diameter of a torque tube.
A second embodiment of the subject invention provides an adjustment means for maintaining optimal vibratory performance of the apparatus as its center of gravity shifts over time. The adjustment means dynamically aligns the effective direction of excitation with the center of gravity of the apparatus by ensuring that the exciter assembly is properly orientated while the device is operational.
Through careful and precise construction steps, the subject invention accomplishes a wide operating frequency range free of any resonant frequency modes. Consequentially, the subject invention can be constructed of much lighter, compact material than present devices requiring massive components and foundations for resonant damage resistance.
Specifically exemplified herein are embodiments for use in the minerals processing industry and, for illustrative purposes, the invention is described hereinafter with reference to this application. However, it is to be understood that the principles underlying the present invention may be applied in other applications such as vibratory screening generally including grading nuts and other food processing applications.